In reference to single wire S.W. Antenna

Here is an example of how they are made and used.

Gnack

Thank You Gnack,

Now I have a better idea what they are all about.

Hey Wayne,

When I built a homebrew RF amplifier to help pick up weak SW signals, I put something together that I have actually seen work when lightening struck somewhere nearby. It consisted of a neon indicator light going from the wire carrying the incoming signal to grount, and in parrallel with this I put a 10K resistor (to bleed off static electricity at voltages below that which would activate the neon indicator) and two 1N4001 diodes, configured so that if one did not conduct the other would (picture two arrows sitting next to each other and pointing in opposite directions.) Again, all of these were in parrallel going from signal to ground, and then I put a .1uF

1000V ceramic disk capacitor going from the signal side of all this to the input of my amplifier (or to the input to the radio, in your case.)

The theory is that the incoming signal will pass by all of this and go through the .1uF capacitor to your radio, but a high-voltage pulse, such as static electricity from someone touching the antenna wire or a nearby lightening strike, will be stopped by the .1uF 1000V capacitor (make sure it is a ceramic disk) and will be discharged by the neon indicator and one or the other of the diodes, with tiny remnants of the pulse being blead off by the 10K resistor. Now, please understand , I am no expert (which someone will no doubt point out) but I actually saw this work one time when I was listening to a SW station while a storm approached. I did not disconnect my equipment before the storm got too close, and finally lightening struck nearby and the neon indicator lit up for an instant. That convinced me it was time! :) And I say that this whole thing worked because even though my antenna picked up the pulse, it was discharged and my equipment did not suffer any negative consequences. I saw the neon indicator light up briefly, and when putting this thing together that was something I never actually expected to see.

In my opinion, the only thing one has to worry about with this setup is losing some signal through the diodes, the PN junctions of which act as small capacitors connecting the signal line to ground. But I have not come up with an answer to that yet. Nothing is perfect...

If this interests you, write to me and I will email the schematic for it.

Hope this helps. Take it easy...

Dave

Just a note: in the absence of an antenna terminal, one could take a couple feet of insulated wire and attach it to the incoming signal line, then wrap a few inches around the whip antenna, effectively coupling the incoming signal to the whip inductively. This is what I usually do, as it makes use of the preamp between the whip antenna and the rest of the radio, giving you better reception and audio.

Dave

Be careful how you're looking at that for your understanding, though. Something that appears to be a DC ground, need not be an AC (RF) ground. So if you had a long wire antenna, and wrapped some turns around the telescopic or whatever, and then grounded the hanging end, you have not grounded the signals, because the turns around the telescopic form an inductance, which serves to couple the signals into the radio, by induction. There are caveats, of course, such as how many turns in your 'coil', the diameter of the winding, the diameter of the wire, and the frequencies of interest. If you look at the schematics for typical RF input stages, particularly with higher frequency receivers, you will often see a coil of just a couple of turns of quite thick wire connected directly to the antenna socket, which you may initially think represents a 'short' to the antenna. If you take it a stage further and start getting up into UHF and low SHF, you will often find a straight piece of wire or even print connected directly across the antenna input. At these frequencies however, that apparent 'short' is actually an inductance, and may well have a reactance of several tens or hundreds of ohms at the frequency of interest.

I have a home-brewed interdigital filter for 1.2GHz ATV lookback notching, and the input and output coupling 'coils' are a single half-turn loop of 1mm wire. The actual 'L' elements are straight pieces of 6mm ally rod.

Arfa

"Dave" wrote in news:9OidnWOJYpeYyhnUnZ2dnUVZ snipped-for-privacy@posted.internetamerica:

....

Unless both ends of that wire are connected, one to the antenna and the other to ground, making this one winding of a transformer, you are actually using _capacitive_ coupling. The wire is one plate of the capacitor, the whip is the other plate and the insulation is the dielectric. The body of the radio forms the plate of another capacitor and the earth forms the second plate of that capacitor.

However, the name of the coupling mode is secondary to the fact that it will provide stronger signals. However, connecting the antenna directly to the whip will also work, and unless you take special precautions to protect against lighting, direct or capacitive coupling are both almost equally hazardous.

7w-110V light bulb antenna -----|------UUU-------|-|------------antenna terminal of rx neon NE2 ^ v diodes opposite pollarity. ground ------|----------------|-|------------ground terminal of rx

Something like the above diagram should work.

I had a nearby lightning strike that vaporized the antenna wire, burned out the lightbulb and scattered pieces of the diodes around. The receiver (a tube type) showed no damages.

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